This invention relates to a novel additive for reducing lost circulation when aqueous drilling fluids are used and a method for reducing such lost circulation. More particularly, the lost circulation additive is an encapsulated lime which will be unable to react with clays in the borehole until reaction is desired to form a cement-like product.
Drilling fluids, or drilling muds as they are sometimes called, are slurries of clay solids used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through pores in the rock matrix surrounding the borehole.
Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation. Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created.
A variety of different substances are now pumped down well bores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Different forms of cellulose are the preferred materials employed. Some substances which have been pumped into well bores to control lost circulation are: almond hulls, walnut hulls, bagasse, dried tumbleweed, paper, coarse and fine mica, and even pieces of rubber tires. These and other prior art additives are described in U.S. Pat. No. 4,498,995.
Another process that is employed to close off large lost circulation problems is referred to in the art as gunk squeeze. In the gunk squeeze process, a large quantity of a gel is mixed in diesel oil and pumped down the well bore. Water injection follows the gel and diesel oil. If mixed well, the water and gel will harden to form a gunky semi-solid mess, which will reduce lost circulation. Problems frequently occur in trying to adequately mix the gel and water in the well.
U.S. Pat. No. 4,481,121 reacts a mixture of lime and wax to produce a soap in an oil based mud. The patent describes a process wherein a montan wax is dispersed within a drilling fluid to provide viscosity and gel strength. Lime is added to the oil based fluid to react with the small amounts of water in the oil based fluid and the montan wax. Partial saponification produces a more thixotropic fluid.
The last few years have witnessed a drastic increase in research on encapsulated products and methods to produce such products. This is particularly so in the pharmaceutical field. And it is now becoming recognized that encapsulation technology may be useful in many other fields.
U.S. Pat. No. 3,971,852 describes a process for encapsulating various fragrance oils such as oils with citrus and spice odors. The oils are encapsulated in a matrix comprised of polysaccharide and polyhydroxy compounds by converting an emulsion of the fragrance oil droplets in a solution of the matrix ingredients to an encapsulated solid state during a spray drying process. The patent also mentions that miscellaneous chemicals can be encapsulated by the invention method such as drilling fluids and waxes.
U.S. Pat. No. 4,269,279 discloses the use of plastic coated magnetic particles in a bead form to increase lubrication for drilling fluids. The encapsulated ferromagnetic particles can be recovered for reuse with a magnetic separator.
An encapsulated invention which has been disclosed for use in boreholes is described in U.S. Pat. No. 4,078,612. The patent describes an explodable material encapsulated in natural gums slurried in a liquid vehicle. The material is pumped into the formation around the wellbore and exploded to increase permeability.
Another U.S. Pat. No. 4,036,301 describes an encapsulated material useful in cementing a well, wherein a cement accelerator is encapsulated in a waxy material and placed within a highly retarded cement slurry. The cement slurry is pumped into the well with the encapsulated accelerator. After proper placement of the cement, circulation is decreased so that the temperature of the cement fluid approaches the bottom hole temperature of the well and melts the encapsulated material, freeing the accelerator which sets the cement.
U.S. Pat. No. 4,362,566 discloses an additional use of encapsulated materials. The patent suggests encapsulating one component of a two or more component adhesive or cement mixture so that the adhesive or cement will not set until the encapsulated component is freed from its reaction-preventive casing.